The price of petrol at the pump is not falling as fast as the price of oil from peak levels over the summer, according to Guardian analysis.

The average pump price is falling by about a penny every five days but still has 5.8p a litre to drop if it is to catch up with recent sharp declines in the price of oil.

A fierce price war - led by Asda, Morrisons and more recently Tesco - has seen prices fall in recent weeks at about twice the rate at which they had inched upwards earlier in the year. Yesterday the average price reached 97.96p a litre.

But Alistair Darling, the chancellor, yesterday demanded that oil companies move faster to pass on the fall in prices to customers on station forecourts.

Speaking on GMTV, Darling said: " A few months ago, the price of crude oil coming out of the ground was about $150 a barrel. Now it's down to about $60. I want to see that reduction passed on to the pumps as quickly as possible."

He said: " People are entitled to see the benefit of that falling price reflected in what they actually pay when they go and fill up their car."

His comments came as Shell smashed analyst forecasts when it reported a profit of $10.9bn (£6.6bn) for the third quarter of 2008, up from $6.4bn the previous year, thanks to the earlier surge in the price of oil.

The fall in the petrol price has helped Labour believe it could yet win the Glenrothes byelection next Thursday.

The oil companies complain that about 70% of the cost of a litre of petrol goes to the government as duty and VAT, so the drop in crude oil prices can only have a limited effect on the cost of filling up at the pump.

Darling is under pressure to help motorists by scrapping the planned rise of 2p a litre in fuel duty, which has been postponed until March 2009.

But Yvette Cooper, chief secretary to the Treasury, joined the attack on the oil firms, telling MPs: "It is important, with world oil prices coming down, [that] those cuts be passed on to consumers because the price increases were certainly passed on to consumers.

"We have seen many petrol companies passing on price cuts, and it would be good to see all petrol companies bringing prices down right across the country. Gas and electric companies could also do more to pass on reductions in oil prices in their fuel bills"

The energy and climate change secretary, Ed Miliband, has already met the energy companies twice to discuss the price of electricity, and has warned he has found signs of lack of competition in the UK energy market.

He is waiting for a report from Ofgem the energy regulator, and has said he would legislate if necessary.

Oil peaked at $147.27 a barrel in July and six days later there followed a peak in the average UK pump prices for petrol at 119.7p a litre.

Since then, fears of a global recession have strengthened, wiping away more than half of the dollar-value of a barrel of oil. The price of oil yesterday reached $65.44 a barrel, a decline of 55.6% from the summer peak.

By contrast, the average UK pump price has fallen 18.2%. A lot of this price goes straight to the chancellor so, stripping out VAT and duty, the underlying price of petrol at the pump has fallen from a peak of 51.52p a litre to 33.02p a litre - a decline of 35.9%.

Part of the reason this underlying pump price fall does not reflect the decline in oil is the weakening of the pound against the dollar.

This means those buying oil in sterling are not able to take the full benefit of the drop in its dollar value.

Stripping out this currency impact, the sterling fall in the value of a barrel is 45.5%. If pump prices were to fall by the same proportion, the average pump price would be 92.16p. This would give motorists filling up a 50-litre tank a saving of £2.90.

Motorists' groups were confident yesterday that the oil price declines would feed through to the pump. "I don't think pump prices are too far behind," said a spokesman for the Automobile Association.

"There was a race among the supermarkets to get down to £1 a litre, led by Asda and Morrisons. Two weeks ago Tesco started to show its muscle with a 3p drop in prices. Others have had to react quickly."

At the pump

• The price of oil has fallen from its peak of $147.27 a barrel in July to $65.44, a drop of 55.6%

• Adjusting to remove the impact of the weakening pound against the dollar, the sterling fall in the price of a barrel of oil is 45.5%

• The petrol price has fallen from 119.7p a litre in July to 97.96p, a drop of 18.2%. Stripping out VAT and duty, the underlying price of petrol has gone from 51.52p a litre to 33.02p, a drop of 35.9%

• So petrol would cost 92.16p a litre today if the price had fallen by the same proportion as oil